Yes, Some D routes are the equivalent of our fastest A roads, and some are dual carriageway too! Most aren't and here Google with Street view is your friend. I 'drove' a route I was wanting to use around Picardie five or six times on Street view and it gave me an awareness of the road conditions and instigated one change of the planned route to avoid a nasty looking stretch of 10 miles which looked heavily used by HGVs going to/from the channel ports.
My experience of cycling in France is that drivers are very much more considerate than in the UK, but the above comments about drink driving is probably true. In North East France they don't seem to go for the 2-3hr lunch breaks where everyone piles into a Cafe, Bistro or Bar for the duration to emerge on the roads driving like idiots. My limited experience of riding further South, Limoges, Bordeaux area suggests it's best to take a break Mid-afternoon from riding while the rush back to work is running.